258 P r anker d . — On the Stricture and Biology of 
is surrounded by a distinct sheath, and shows a tendency to become a con- 
centric structure by the strong incurving of the arms. Fig. 4, PL XX, shows 
a case where three almost complete steles were thus formed (cf. Diagram 5, 
p. 256), and though there is considerable variation in structure at this region, 
there is always more or less of a polystelic phase. At a lower level, the 
large branch traces, entirely concentric in structure, fuse with these central 
steles, and in so doing form a complete ring of xylem, surrounded by 
phloem and bounded externally and internally by a pericycle and endodermis, 
Text-FIG. 3. Part of a transverse section across the aquatic stem near the base of the 
inflorescence. ( x 108.) 
or in Jeffrey’s terminology, an ectophloic siphonostele (PI. XX, Fig. 7, and 
Diag. 3, p. 256). In young plants, where the inflorescence is in bud, the 
internal endodermis is almost as regular as the external, exhibiting the 
Caspary dots on the radial walls (Text-fig. 3) in transverse section ; but in 
the mature plant the thickening spreads over the whole of the cell-walls, 
and even extends to the walls of adjacent cells (Figs. 7 and 8). 
As the pericycle has no special differentiation, it would be difficult to 
be sure of the presence of this layer internally if it were not that it frequently 
